1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiplex Access) receiving apparatus for use with a mobile radio communication system of the CDMA type.
2. Description of the Related Art
A CDMA receiving apparatus for use with a mobile radio communication system for a portable telephone set or the like of the CDMA type conventionally has a general construction as shown in FIG. 8. Referring to FIG. 8, the CDMA receiving apparatus shown includes a plurality of FINGER processing sections 1 (FINGER processing sections 1.sub.1 to 1.sub.n) a RAKE composition section 2, a decoding section 3 which performs decoding processing including error correction of a reception signal after detection, and a synchronization detection and phase tracking section 4. In FIG. 8, reference symbol A denotes a reception signal before de-spreading, B a delay amount indication signal indicating a timing of de-spreading detected and instructed by the synchronization detection and phase tracking section 4, and C a reception signal after detection. In the receiving apparatus shown in FIG. 8, in order to process each reception signal A of a plurality of paths (that is, multi-paths), the plural number of FINGER processing sections 1.sub.1 to 1.sub.n are provided corresponding to the paths. The term "FINGER" of the FINGER processing sections signifies a signal like a finger, and such processing sections are called FINGER processing sections since they process the reception signal A which is a finger-like signal. Meanwhile, the RAKE composition section processes signals outputted from the FINGER processing sections like a rake.
FIG. 9 shows a block diagram of a construction of the FINGER processing sections 1.sub.1 to 1.sub.n in the CDMA receiving apparatus. Referring to FIG. 9, the FINGER processing section 1 shown includes a de-spreading section 5 and a channel estimation section 6. The de-spreading section 5 includes correlators 8 (8.sub.1 to 8.sub.3). A correlation magnitude D is outputted from each of the correlators 8 of the de-spreading section 5 to de channel estimation section 6. The channel estimation section 6 includes a channel estimator 12, and a detection section 13 which interpolates symbol positions of the reception signal A based on an estimate channel vector obtained by the channel estimator 12.
A reception signal A is a modulated signal whose signal spectrum is spread by a spread code when it is transmitted from the transmission side. Consequently, upon reception of the reception signal A, the synchronization detection and phase tracking section 4 modulates the reception signal A by successively displacing the phase of a de-spread code (same code as the spread code but inverse in polarity) to determine correlation magnitudes. Then, those of the correlation magnitudes which are higher than a threshold value designated in advance are determined, and each phase of the de-spread code corresponding to the determined correlation magnitudes is indicated to the de-spreading sections 5 of the FINGER processing sections 1.sub.1 to 1.sub.n that is, n phases of de-spread code (reception delay amounts) corresponding to comparatively high ones of the determined correlation magnitudes which are designated in the descending order.
Operation of the CDMA receiving apparatus is described with reference to FIGS. 8 and 9.
A reception signal A is a modulated signal whose signal spectrum is spread by a spread code when it is transmitted from the transmission side. Consequently, upon reception of the reception signal A, the synchronization detection and phase tracking section 4 modulates the reception signal A by successively displacing the phase of a de-spread code (same code as the spread code but inverse in polarity) to determine correlation magnitudes. Then, those of the correlation magnitudes which are higher than a threshold value designated in advance are determined, and each phase of the de-spread code corresponding to the determined correlation magnitudes is indicated to the de-spreading sections 5 of the FINGER processing sections 1.sub.1 to 1.sub.n that is, n phases of de-spread code (reception delay amounts) corresponding to comparatively high ones of the determined correlation magnitudes which are designated in the descending order.
The correlators 8 of the de-spreading section 5 modulate the reception signal with the de-spread code at the respective designated timings (phases) to de-spread the reception signal A, and outputs correlation magnitudes of the de-spread reception signal A to the channel estimator 12 and the detection section 13. Each reception signal A of the multi-paths can be separated by de-spreading the reception signal A at timings corresponding to the individual paths.
In this instance, the channel estimator 12 estimates a displacement in phase caused by fading and outputs the estimated displacement to the detection section 13. The detection section 13 interpolates a symbol position of the de-spread reception signal using a vector estimated by the channel estimator 12 and outputs a resulting signal as detection data C.
The detection data C detected by the n FINGER processing sections 1.sub.1 to 1.sub.n in this manner are sent to and added by the RAKE composition section 2, and a result of the addition is outputted to the decoding section 3. The decoding section 3 performs decoding processing including error correction to the reception signal from the RAKE composition section 2.
FIG. 10 shows a block diagram of a construction of a CDMA receiving apparatus disclosed in Japanese Patent Laid-Open Application No. Heisei 8-256084. Referring to FIG. 10, the CDMA receiving apparatus shown includes an antenna 101, a mixer 102, an oscillator 103, a correlator 104, a PN (Pseudo Noise=de-spread code) load signal control circuit 105, PN generators 106 to 108, a delay difference detection circuit 109, a correlation magnitude detection circuit 110, de-spreading circuits 111 to 113, delay lock loops 115 to 117, demodulation sections 118 to 120, delay correction circuits 121 to 123, multiplication circuits 124 to 126, an addition circuit 127, and a normalization circuit 114.
In operation, a spread signal received by the antenna 101 is converted into a base band signal by the mixer 102 and the oscillator 103, and the base band signal is outputted to the correlator 104, de-spreading circuits 111 to 113 and the demodulation sections 118 to 120. The correlator 104 detects a correlation of the reception signal to a PN code (de-spread signal) similar to that used on the transmission side while successively displacing the phase of the PN code to determine correlation magnitudes corresponding to a plurality of paths (in the arrangement shown in FIG. 10, the correlator 104 includes three correlators).
The PN load signal control circuit 105 selects three phases corresponding to the three highest correlation magnitudes obtained by the correlator 104 in descending order and outputs the three phases as PN load signals LDn (LD 1 to LD3) to the PN generators 106 to 108. The PN generators 106 to 108 respectively generate PN signals PNn (PN1 to PN3) synchronized with the PN load signals LDn and clock signal from the delay lock loop circuits 115 to 117. Consequently, PN signals synchronized in phase with the multi-paths can be obtained. The PN signals PNn form the PN generators 106 to 108 are supplied to the demodulation sections 118 to 120, respectively, and the correlation magnitude detection circuit 110. The demodulation sections 118 to 120 demodulate the reception signal based on the PN signals PNn. In this instance, the top bits of the PN signals PNn are supplied to the delay difference detection circuit 109. The delay difference detection circuit 109 detects phase differences of the PN codes PNn based on the received top bits of the PN signals PNn and controls the delay correction circuits 121 to 123 to correct the phase differences corresponding to the delay differences of the multi-paths.
The correlation magnitude detection circuit 110 determines correlation magnitudes using the reception signal and the PN codes PNn supplied from the PN generators 106 to 108 to the demodulation sections 118 to 120. The multiplication circuits 124 to 126 multiply the demodulation signals from the demodulation sections 118 to 120 received through the delay correction circuits 121 to 123 by the correlation magnitudes of the correlation magnitude detection circuit 110 as weight coefficients and output results of the multiplication to the addition circuit 127, by which the results of the multiplication are added.
Even the conventional CDMA receiving apparatus described above with reference to FIGS. 8 and 10 can accurately perform phase tracking for a spread reception signal whose reception delay amount (that is, timing of de-spreading) varies in a period substantially equal to the processing time of the synchronization detection and phase tracking section. However, the conventional CDMA receiving apparatus has a subject to be solved in that it cannot accurately perform phase tracking for another spread reception signal whose reception delay amount varies in a period shorter than the processing time of the synchronization detection and phase racking section. In other words, since the synchronization detection and phase tracking section searches for effective paths from within the reception signal and tracks the phase as a detected path, it performs processing to average the obtained correlation magnitudes in a period of a frame of the signal. Accordingly, the conventional CDMA receiving apparatus cannot accurately perform phase tracking to the reception signal when the reception delay amount varies in a period shorter than the average processing time of the synchronization detection and phase tracking section.